The conventional organic multilayer devices, such as organic photovoltaic (OPV) devices, e.g. organic light emitting diode (OLED) devices, consist of a stack of layers of different material layers. A common technique exists in the application of one or more small molecule layers. The individual layers have specific functions such as hole transport layers, electron transport layers or emissive layers. The conventional method to deposit a small molecule layer in a multilayer stack for OLED/OPV devices is to evaporate each layer in a high vacuum evaporation tool. As may be appreciated, the use of a vacuum renders the manufacturing method to be time and cost consuming, as well as cumbersome.
A potentially advantageous alternative to the above method of evaporating each layer, would be to print a multilayer stack from solution printing. A solution printing process for manufacturing semiconductor elements typically consists of regular printing methods such as jet printing or slot dye coating, sometimes supported with added technology such as to create desired semiconductor patterns with necessary accuracy and complexity. The printing method allows for printing desired semiconductor materials which are dissolved in solvents, and enables to print such materials layer for layer. Such a method may also be used for printing organic semiconductor materials, e.g. for creating organic devices, such as photovoltaic (OPV) devices, e.g. organic light emitting diode (OLED) devices.
A known problem in multilayer deposition of small molecule layers from solutions, such as the above printing method, is the fact that when a layer is deposited onto another layer, part or most of the material in the underlying layer will dissolve in the solvent used for the deposited layer. This makes multilayer processing from solution very difficult.
One method to solve this problem is the use of small molecules which dissolve in different (orthogonal) solvents, so that a second solution will not dissolve the first layer. However the resulting small molecules are not necessarily optimal for their purpose in an OLED or OPV stack, and it is a material-dependent rather than generic solution to the problem.
Another possible solution is to perform a treatment step on the underlying layer to treat the small molecules of the underlying layer after deposition in such a way that they do not dissolve anymore. Such a treatment may exist in annealing (usually at around 180° C.) or lowgrade polymerisation or cross-linking (of the material itself or of a host matrix). However annealing at high temperatures is not possible when plastic substrates are used and the agents used for cross linking can damage the devices, e.g. resulting in poor lifetimes of the OLED's.
Another possibility is to use a liquid buffer layer of a material such as ethylene glycol. Such a buffer is deposited onto the first organic layer. The second organic layer is then deposited on top of the buffer, and the liquid buffer slowly evaporates through the deposited top layer as it dries. This prevents intermixing with the lower organic layer with the solvent of the deposited layer. However, this method has only been shown to work for very small samples (few cm2) and depends a lot on the wetting or surface energies of the materials.
Another alternative is to use very high molecular weight materials which would take a long time to dissolve when a second organic layer is deposited from solution on top of them. However it is difficult to solution process high molecular weight materials, and as a result it is cumbersome to perform this accurately. Furthermore, this is another alternative that limits the choice of materials for the designer. The high molecular weight materials are not necessary the best candidates for the design of the semiconductor device at hand. For example, for OLED's such materials are not desirable for the emitting layer as the polymer host materials usually have a triplet energy which is low enough for allowing back transfer from the blue phosphor to the polymer host, resulting in sub-optimal performance.
Lamination of two halves of a multilayer semiconductor element is another “dry” technique which has been used to make multilayers. The adhesion of the two layers and the exclusion of any defects is however critical here, increasing the chance on defects and thus decreasing the yield of the process. Moreover, if this technique is to be used multiple times within a device it would also involve the use of many carrier foils.